Overload and short circuit protector for transistor voltage regulator



Feb. 26, 1963 D. L. DECKER 3,079,543

OVERLOAD AND SHORT CIRCUIT PROTECTOR FOR TRANSISTOR VOLTAGE REGULATORFiled March 31, 1960 SIGMA:

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INVENTOR. 0/: VIP A. Dfit'AE/P BY Q/(w 14w OVERLOAD Ann Iowa Filed Mar.31,-196tt, SenNo. 18,922 1 Claim. (U. 323-22) This invention relates toan overload and short circuit protector device and more particularly toa device for use in voltage and current regulators of the type utilizingsemi-conductors to protect the components of the regulator againstdamage due to heavy overloads or shorts in the output circuit of theregulator.

Voltage and current regulators are well knownin the art and in the pastcommonly utilized at least one power amplifying tube to provide thedesired regulated output. While it was desirable to protect theregulator, and es pecially the power tube, against damage, whichsometimes required the use of protector devices, thenecessity of suchaprotector device has been accented in recent years due to thedevelopmentand use of semi-conductors in voltage and current regulators.This is due'to the fact that a semi-conductor is far more susceptible tosevere damage due to the heat produced during excessive conduction ofcurrent than is a vacuum tube. 7

various protector devices have heretofore been suggested and utilized toprotect the components, and es pecially the semi-conductors, used in thevoltage and current reguilato-r against damage due to a short in theoutput circuit since this obviously would cause heavy current conductionthrough the regulator and damage its components severely in a very shorttime.

Providing a protector device that merely guards against a short in theoutput circuit, however, is not sufficient, of itself, to adequatelyprotect a voltage and current regulator since a short'in the outputcircuit is only one possible occurrence which can cause damage to theregulator. An additional, and very real possibility of damage, alsoexists in the event that a heavy overload is connected into the outputcircuit of the regulator, since this too will cause excessive currentconductiontherethrough.

It is therefore an object of'this invention to provide simple, yetreliable means to protect a voltage and current regulator against damagedue to either a short or a heavy'overload in the output circuit of theregulator.

It is another object of this invention to'provide means responsive tothe regulator output voltage to bias the power amplifier of theregulator to cutoff whenever a heavy overload or short appears in theoutput circuit of the regulator to thereby protect said regulatoragainst damage due to excessive current flow therethrough.

More particularly it is an object of this invention to provide protectormeans including a transistor normally biased to cutofi so that saidmeans is operable only when the regulated output voltage of theregulator drops below apredetermined and preset level to thereafterpreclude further conduction of current through the regulator.

'It is still another object of this invention to provide protector meansoperable to preclude current flow through the regulator after a heavyoverload or short is connected into the output circuit of such aregulator and to maintain said regulator in its inoperative state untilsuch time as the heavy overload or short is removed from the outputcircuit and the input signal to the regulator is thereafter removed andthen reapplied.

With these and other objects in view which will become apparent to oneskilled in the art as the description proceeds, this invention residesin the novel construction, combination and arrangementcf partssubstantially as hereinafter described and more particularly defined byarrests Patented Feb. 26, 1963 the appended claims, it being. understoodthat such changes in the precise embodiments of the herein disclosedinformation may be submitted as come within the scope of the claims.

The accompanying drawings illustrate two complete examples of twoembodiments of the invention constructed according to the best mode sofar devised for the practical application of the principles'thereof, andin which:

FIGURE '1 is a schematic circuit diagram of a voltage and currentregulator having the protector device-of this invention incorporated andillustrated as a part thereof; and" FIGURE 2 is a schematic circuitdiagram of a modified voltage and'current regulator having. amodifiedprotector device incorporated and illustrated as a part thereof.

Referring now to the'd'rawings in'which'like numerals have been used toidentify like characters throughout, the numeral 3' designates generallya voltage and current regulator, which may be of conventional type, inwhich the protector device 4" of this invention may be utilized toprotect the regulatoragainst damage should a short circuit or a heavyoverload be" connectcd'into its output circuit.

As shown inPIGUR'E 1, Voltage and current regulating device 3 mayinclude a power amplifier 6, which is preferably a transistor, andsignalsampling means'7 to regulate the conduction of transistor 6 bycontrolling the bias thereof.

As shown in the drawings, transistor 6 may be of'the PNPtype' (as shownin FIGURE 1) or of the NPN' type (as shown in FIGURE 2 and designatedwith the numeral 1%) so long as other circuitry of the regulator isclfianged accordingly as brought outmorte'fully hereina ter.

As shown in FIGURE 1, transistor 6 may have its emitter connected withthe positive output of a source of unregulated direct current signal 8by means of a lead it its collector connected with a variable outputcircuit load 12 by means of regulated output circuit lead 14, and itsbase connected with the negative output of source 8 through biasresistor 16' by means of leads 18 and 15 which are connected to ends 2%and 21, respectively of bias resistor'16. To complete the outputcircuit, load l2 may also be connected to lead 19 by means of lead 22.If desired, stabilizing-capacitor 23" may be connected acrossoutput'leads l4 and 22 to prevent'oscillation.

Signal sampling means 7, as above-stated, regulates thebias ontransistor 6' to therebycontrol the conduction of-the same so that aregulated output is supplied by the regulator. As shown by the drawings,signal sampl-ing means '7 includes a voltage divider 2E which mayinclude a fixed resistor 26 and a variable resistor 27 seriallyconnected tbetween leads 14 an'd22. As shown in FIGURE 1, the variabletap-23 of resistor 27 may be connected to the b'aseofatransistor'iidof'the NPN type through current limiting resistor Slanddiode 32, which is, or course, connected so that low resistance-isofiered to base current flow.

The emitter of'transistor"3tt is connected'to leads 19 and 22 through abreakdown junction or Zener diode 35, which diode is also connected toinput lead 10 through bias resistor 36. Diode 35is orientated so thatthe voltage applied to the emitter of-transistor 30 is maintained at thebreakdown voltage of the diode.

The collector of transistor 36 isconnected tothe base of a 'PNP' typetransistor 4t through cur-rent limiting resistor 41 while the collectorof transistor 40 is, in'turn, connected to-side 200i resistor 16,-which, as'brought out hereinbefore, provides bias for transistor 6. Theemitter of transistor 4% is directly connected to input lead'luby meansof leads 45 and 46, and-resistor 1'6 is'therefoi'e aosaeae d) also inthe output circuit of transistor 4t Forward bias for transistor 4% isprovided by resistor 48 which connects the emitter and base of saidtransistor.

The protector device 4 of this invention includes a voltage divider 53which may include fixed resistance 54, and variable resistance 56, thevariable tap 57 of which may be directly connected to the base of a PNPtype transistor 69, as shown in FIGURE 1. The emitter of transister asmay be connected with input lead it} through resistor 52 and connectingleads 46 and 63. A breakdown junction or Zener diode 64 is connectedbetween leads 19 and 63 to maintain the voltage of the emitter oftransistor ed at the breakdown voltage of the diode. In addition, acapacitor 65 is connected in parallel with diode 6d, and this capacitoralong with resistor 62 provides an RC. time constant the purpose ofwhich is hereinafter brought out.

The collector of transistor 6%} is connected to the base of transistor30 through limiting resistor 66 and due to the orientation of diode 32,collector circuit current must be applied to the base of transistor 3%,the result of which is to bias transistor 6 to cutoff as brought outhereinbelow. in operation, the voltage and current regulator shown inFIGURE I normally operates in the following manner. Assuming that theload 12 is increased by an amount not considered excessive, the voltagepotential between leads 14 and 22 will decrease thus decreasing thevoltage drop across voltage divider 25. This means that a lower, or morenegative, voltage is impressed upon the base of transistor 39, and sinceit is a transistor of the NPN type, conduction inthe base circuitdecreases causing a corresponding decrease in conduction in thecollector circuit. This decrease in current flow, of course, causes adecrease in the voltage drop across bias resistor 48 which is in thecollector circuit of transistor 30. The decreased voltage drop acrossresistor 43 causes the emitter of transistor at} to become less positivewith respect to the base so that conduction in the base-emitter circuitof PNP type tran sister 40 is decreased. The collector current oftransistor 4t) will, of course, likewise be decreased, and sinceresistor 16, which provides bias for transistor 6, is also in thecollector circuit of transistor 40, there will be a decrease in thevoltage dropped across this resistor. This decreased voltage drop acrossresistor 16 is impressed upon the input circuit of transistor 6 so thatthe base of transistor 6 becomes less positive, or more negative, thanthe emitter, and results in increased conduction by transistor 6.

When the output voltage of the regulator is substantially returned toits former level, the bias of transistor d, which is controlled by theoutput level, will, of course, be altered accordingly to decrease theconduction of transistor 6 to maintain the output at a predeterminedregulated level.

The protector device of this invention is normally inoperative duringthe above described normal operation of the regulator since thisprotective network is needed only in case of a heavy overload or a shortin the output circuit. This is accomplished by biasing transistor dtl tocutofi for normal operation by adjusting variable tap 57 of resistor 56(which is a part of voltage divider 53) so that a higher positivevoltage is impressed upon the base of PNP type transistor do than ispresent on the emitter. Thus the protector device can be preset tobecome operative at any predetermined output potential by adjustingvariable tap 57.

From the foregoing it will be appreciated that the protector device ofthis invention will be inoperative until the potential between outputleads lid and 22 decreases to a value that causes the voltage impressedupon the .base of transistor as to be more negative than the emittervoltage.

When, for example, output circuit load 12. is increased to such anextent that it constitutes a heavy overload, the potential between leadsl4 and 22 will be decreased causing a corresponding decrease in voltagedrop across dividers 25 and 53. It is to be noted that the same decreasein voltage drop across these dividers occurs in the case of a shortcircuit although it is usually much more pronounced and acts to shortout the dividers completely. When such a heavy overload or short appearsin the output circuit of the regulator, the regulator normally tries tosupply the necessary voltage to return the output to its former level.This, of course, can cause excessive conduction and severe damagethrough excessive heat to the components of the regulator and especiallyto the power transistor. The protector device of this invention preventsthis excessive conduction in the -following manner.

When a heavy overload or a short first appears in the output circuit,transistor 6d, which, as brought out hereinbeiore is normally biased tocutoff, will immediately start to conduct because of the decreasedvoltage impressed upon its base to make it negative with respect to theemitter. Collector current from transistor so is applied to the base oftransistor 3h causing the base of NPN type transistor 34? to become morepositive with respect to the emitter. This causes transistor 3d toconduct at an increased rate and causes a greater voltage drop acrossresistor 48, which, in turn causes the emitter of PNP type transistor 40to become more positive with respect to the base. This causes transistor4% to conduct at an increased rate and causes a greater voltage dropacross resistor 21 which, in turn, causes a greater voltage to beimpressed on the base of transistor 6 biasing the same to cutoff andthereby rendering the regulator inoperative.

When protector device 4 is caused to operate and bias transistor 6 tocutofi, the voltage regulator will remain inoperative until such time asthe input voltage is removed from the regulator. When this voltage isreapplied the R.C. time constant provided by resistor 62 and capacitor65 is such so as to allow the output to build up and bias transistor 64]to cutoff before full voltage is applied to the emitter of transistor69. If, of course, the heavy overload or short circuit has not beenremoved, however, the protective device will again immediately biastransistor s to cutoff and render the regulator inoperative.

It is to be appreciated that the protector device of this invention maybe modified by using a transistor of the NPN type, if the circuitry ofthe voltage and current regulator in which it is utilized so requires,without departing from the spirit or scope of this invention.

FIGURE 2 illustrates, for example, a voltage and current regulator 193utilizing a transistor of the NPN type in place of PNP type transistors6 and 40 and a PNP type transistor in place of NPN type transistor 3b asshown in FIGURE 1. The protector device 1% in this case may utilize atransistor of the NPN type in place of the PNP type transistor 60 asshown in FIGURE 1.

As shown in FIGURE 2, a negative lead 11% is connected to the emitter oftransistor 1% while positive lead 119 is connected to the emitter oftransistor 1% while positive lead 119 is connected to the base oftransistor 1%. In addition, lead its is a negative output circuit leadand is connected with the collector of transistor ltld, while lead 12 2is the positive output circuit lead and is connected to lead 119. It cantherefore be readily appreciated that the circuit will operate in muchthe same manner as that of FIGURE 1, but the voltage polarities will bereversed. For this reason, Zener diodes 35 and 64 must, of course, bereversed and the same is true of diode 32..

in operation, when an increased load, not considered excessive, isapplied to output circuit load 12, the voltage potential between leads Al and 122 will be decreased. This, of course, causes a decrease involtage drop across voltage divider 25 and causes a decrease in thevoltage appliedtothe base of PNP type transistor 13d. it is tobe-remembered, however, that the emitter of transistor 13% is alsonegative so that a decrease in negative voltage on the base decreasesthe difference in potential between the emitter and base. This causes adecrease in conduction in the input circuit and will, therefore,decrease current conduction in the collector circuit. This causes adecrease in the voltage drop across bias resistor 48 and because of thepolarity of this voltage drop makes the emitter of NPN type transistor140' less negative with respect to the base and causes decreasedconduction in its output circuit. This, in turn, causes a decreasedvoltage drop across bias resistor 16 and, due to the polarity of thisvoltage drop, the voltage impressed upon the base of NPN type transistor106 makes the base less negative, or more positive, with respect to theemitter and causes increased conduction in the output circuit to returnthe voltage potential between output leads 114 to 122 to the desiredlevel.

If, however, a short or a heavy overload is connected into the outputcircuit of the regulator, protector device 104, which has been modifiedby replacing the PNP type transistor shown in FIGURE 1 with a transistorof the NPN type as brought out hereinabove, will conduct to biastransistor 106 to cutotf and render the regulator inoperative. This isaccomplished by applying the collector current of transistor 160 to thebase of transistor 130 in the same manner as that described inconnection with the protector device shown in FIGURE 1. This impressedvoltage causes increased conduction in PNP type transistor 130, which inturn causes increased conduction in transistor 140. The resultingincreased voltage drop across resistor 16 impresses a more negativevoltage on the base of NPN type transmitter 106 and causes the same tobe biased to cutoff.

As described in connection with FIGURE 1, the regulator shown in FIGURE2 will likewise remain in a nonoperative state until such time as theinput signal from source 9 is removed from the input leads 110 and 119and then reapplied since the emitter of transistor 160 will remain morenegative than the base until the signal is first removed from thisemitter.

While two specific examples have been shown and illustrated herein toadapt the protector device of this invention to a specific voltage andcurrent regulator, it is to be appreciated, of course, that theprotector device of this invention might be modified to meet otherspecific circuitry requirements without departing from the spirit orscope of this invention.

In view of the foregoing it should be obvious to those skilled in theart that the protector device of this invention provides simple yetreliable means for protecting a voltage and current regulator againstdamage to its components from excessive conduction through the regulatordue to a heavy overload or a short in the output circuit of theregulator.

What is claimed as my invention is:

A transistorized voltage and current regulating device of the typehaving output overload and short circuit protector means and requiringno operating power other than that from an unregulated D.C. source, saidregulating device comprising: power amplifying means having an inputcircuit and an output circuit, said input circuit including resistancemeans for biasing said power amplifying means and said output circuitincluding voltage divider means; means adapting said input circuit forconnection with an unregulated D.C. power source; means adapting saidoutput circuit for connection with a load so that said load is inparallel with said voltage divider means; output voltage sampling meansconnected between said voltage divider means and said resistance meansto determine and regulate the bias on said power amplifying means; atransistor having an input electrode, a control electrode, and an outputelectrode; a Zener diode connected with said input circuit and with saidinput electrode for assuring that a constant voltage is applied to saidinput electrode; means for applying direct voltage from said voltagedivider means to said control electrode to normally bias said transistorto cutoff, said transistor remaining in a nonconductive state until aload connected with said output circuit draws excessive current causingsaid transistor to conduct, said transistor then remaining in saidconductive state until the power from the unregulated source is removed;means connecting said output electrode with said voltage sampling meansfor coupling an output signal thereto while said transistor is in aconductive state to bias said power amplifying means to cutoff; and anRC. network including a capacitor in parallel with said Zener diode anda resistor having one end connected to said input electrode and theother end connected to said input circuit whereby said R.C. networkprevents the application of breakdown voltage to the input electrode ofsaid transistor for a predetermined time interval immediately afterpower from the unregulated source is coupled to said regulating deviceto thereby allow cutofi bias to first be impressed upon the controlelectrode of said transistor.

Harrison Dec. 1, 1959 Norris et a1. Jan. 26, 1960

